1 Answer
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Let $\varphi\colon\mathbb Z^2\to \mathbb Z$, $\varphi(x,y)=x-y$. Then $\varphi$ is a homomorphism. Since $\ker\varphi = \langle (1,1)\rangle$ and $\varphi$ is surjective, the first isomorphism theorem gives the answer.